Optical disks typified by a DVD-RAM have become widespread as information recording media on which massive amounts of digital data such as AV (Audio Video) data, computer data and the like can be recorded. In order to secure the sound distribution of digital works recorded on such information recording media, there is a demand for a technique that prevents digital works on an information recording medium from being duplicated illegally to another information recording medium.
One conventional technique for preventing illegal duplication is such that in accordance with a pseudo random number sequence that is generated inside a given apparatus, an edge in a tangential direction of a recording mark to be recorded on an optical disk is displaced to a position advanced or delayed by a constant slight amount, thus recording sub-information. Such a technique is proposed as a method of recording key information, as sub-information, for decoding encrypted contents recorded on an optical disk (see, for example, JP2001-357533 A).
FIG. 23 is a circuit block diagram schematically showing a configuration of a conventional information recording apparatus that records sub-information by displacing an edge in a tangential direction of a recording mark by a constant slight amount.
In FIG. 23, upon receipt of an initialization timing signal (INIT) from a main information recording part (not shown), which indicates a timing for recording sub-information, a pseudo random number generator 2301 initializes a pseudo random number sequence using an initial value 2302 that has been stored in advance secretly inside an apparatus, generates a pseudo random number sequence (PRS) in synchronization with a byte clock from the main information recording part (not shown), and outputs it to an exclusive OR circuit (XOR) 2303.
The XOR 2303 performs an exclusive OR operation of sub-information (contents encryption key) (SI) to be recorded and the pseudo random number sequence from the pseudo random number generator 2301 so as to output a sub-information recording control signal (SIW) to a phase encoding (PE) modulator 2304.
In the PE modulator 2304, the sub-information recording control signal SIW outputted from the XOR 2303 is subjected to PE modulation so as to generate a phase modulation control signal (PMC), and the phase modulation control signal (PMC) is transmitted to a recording channel. Based on this phase modulation control signal PMC, modulation is performed in which an edge of a recording signal for recording main information is advanced or delayed, and thus the contents encryption key is recorded as sub-information.
Furthermore, in an information reproducing apparatus, sub-information is reproduced based on a correlation of a phase error signal obtained as a result of detecting whether an edge of a reproduced signal of main information is advanced or delayed with a pseudo random number sequence generated based on an initial value that is the same as used in the case of the information recording apparatus.
In the above-described technique, since sub-information (contents encryption key) is superimposed as jitter of a reproduced signal, unless an accurate pseudo random number sequence can be generated, it is impossible to record or reproduce sub-information appropriately. This allows digital works recorded on an optical disk to be protected from illegal duplication.
However, in this technique, it is required that all apparatuses use a standardized pseudo random number generator and an initial value thereof Because of this, particularly, once the initial value is publicized, proof of every illegal duplication might be lost. Thus, the initial value should be managed strictly.
Furthermore, in considering such information recording/reproducing apparatuses that record and reproduce main information and sub-information, once an area in which sub-information is recorded is revealed, it hardly can be said that there is sufficient proof of an act of recording encrypted contents and an encryption key of the encrypted contents recorded as sub-information entirely onto another optical disk.